The invention relates to a planar optical waveguide and to a method of spatially selectively increasing the refractive index in a glass.
Planar optical waveguides of the type defined are known from an article by G. D. Maxwell entitled "Photosensitivity & Rare-Earth Doping in Flame Hydrolysis Deposited Planar Silica Waveguides", Proceedings of the International Society for Optical Engineering (SPIE), Vol.2695, p. 16-29. It is a common feature of all the glass waveguides described therein that the photosensitivity of the glass used is increased by suitable measures. Photosensitivity refers to the property of a material to react to irradiation with light of a suitable wavelength by a change in its refractive index of greater or lesser duration. Utilizing this effect, regions having an increased refractive index compared to the adjoining regions are formed in the optical waveguides by locally limited irradiation with UV light. By appropriately arranging these regions, for example a Bragg grating can be formed in the planar optical waveguide. Bragg gratings play an important role in transmission devices based on the principle of optical wavelength multiplexing.
The silicate glass used to produce optical waveguides has an extremely low inherent photosensitivity. Therefore without special treatment only a very small change in the refractive index can be obtained when the glass is exposed to a UV laser. The above mentioned article by G. D. Maxwell specifies several measures whereby the photosensitivity can be increased.
One measure consists in exposing the glass to a reducing atmosphere during sintering. However this leads to increased optical losses in the sintered glass for the wavelengths normally used in optical transmission technology. In the case of another measure also used in optical fibres, the glass is exposed to a hydrogen atmosphere at high pressure. This process, which is also referred to as "hydrogen charging" extends over several days; moreover, although the photosensitivity thereby attainable is relatively high, it is not very durable. Therefore the change in refractive index inducible by means of the irradiation also is not durable, for which reason optical components produced in this way must be more frequently readjusted by repeated UW irradiation. In the case of another known measure known as "flame brushing" the glass is treated with a hot oxyhydrogen flame. An excess of hydrogen in the flame causes a reaction presumably comparable with that which occurs in the case of hydrogen charging. The relevant details are given for example in an article by S. Gujrathi entitled "Photosensitivity in Phosphorus-Doped Silica Glass and Optical Waveguides", Appl. Phys. Lett. 65 (4), Jul. 25, 1994. However, similarly as in the case of hydrogen charging, here too the increase in photosensitivity is not very durable.